Various methods are under development that will increase the antireflection property of a substrate that can then be used in functional units, such as a thin-film photovoltaic cell, in optical devices or optoelectronic devices. For the substrate in optical devices or optoelectronic devices, a glass substrate is widely used since it has a low cost, high transmittance of visible light and high resistance to mechanical scratches and an excellent barrier property to water and oxygen.
When an antireflective (AR) coating is formed on the glass substrate in order to impart the glass substrate with an AR property, it needs to satisfy the following conditions: (1) the thickness of the AR coating be ¼ or less of the wavelength of incident light, and (2) the refractive index of the AR coating be about 1.22 or less.
The refractive index is expressed by nc=(nang)1/2, where nc, na and ng are the refractive indices of the coating, air and glass, respectively. Since the refractive index of glass is typically about 1.50, the ideal value of nc is 1.22. However, dense thin-film materials that have a low refractive index of about 1.22 are not available.
Although examples for the low-refractive-index material that is used in optical devices may include silica (SiO2), calcium fluoride (CaF2) and magnesium fluoride (MgF2), their refractive indices are 1.46, 1.44 and 1.39, respectively, which fall short of the ideal refractive index.
An alternative option is to use a coating having a porous structure, in which a coating material and air form a composite layer. In this case, the refractive index neff of the composite layer is a value between the refractive index of the coating material and 1, and can be controlled by changing the fractional volume ratio.
Several methods for realizing such a porous coating on the glass substrate have been implemented. These methods involve sol-gel, a polymer and/or oxide colloid monolayer or multilayer, selective etching and dissolution of separated copolymers. However, polymers are incompatible with existing fabrication methods for electronic devices since they have the drawback of thermal, mechanical and chemical instability and are sensitive to high temperature and plasma. In addition, colloid has poor mechanical endurance due to weak bonding to the glass substrate. The other methods using vacuum processing and lithography have the drawback of high costs. Furthermore, since an additional coating layer must be formed on the glass in order to realize an antireflection property, the number of processes and the cost increase. In addition, intensive efforts are required for the selection of materials. Therefore, a method that is inexpensive, simple and applicable to a large area is important in order to fabricate an antireflective coating that exhibits high performance, is extensive, and is endurable.
In addition, during practical operations, especially in devices which operate outdoors, dust or impurities may be attached to the exposed surfaces, thereby degrading the performance of these devices. For this reason, the combination of self-cleaning and an AR property is desirable for use in outdoor photovoltaic devices, displaying devices, self-cleaning windows and vehicle windshields. Although the surface must be super-hydrophobic or super-hydrophilic for self-cleaning, the AR property and self-cleaning are competitive properties in practice. Therefore, the two properties are not realized at the same time.
According to the Wenzel model and Cassie-Baxter model, high surface roughness is required in order to satisfy super-hydrophobic or super-hydrophilic requirements. However, such a rough surface typically causes severe light scattering, and thus the AR property cannot be realized. Photo-responsive coatings (TiO2, ZnO or the like) can also lead to super-hydrophilicity. However, their refractive indices are too high, which is problematic.
The porous structure can be an attractive option because it has been used to manufacture AR coatings, as well as super-hydrophobic and/or super-hydrophilic layers. However, although it is crucial to reduce light scattering by controlling the pore size in order to realize a highly transparent and self-cleaning AR coating, the two properties have not been concurrently realized up to the present.
It is known that reflection from a glass surface can be reduced by removing leachable components from the glass so as to leave a skeleton layer. However, all such chemical treatments require the use of processes and complex acidic solutions that are optimized in practice for one specific type of glass, which is problematic. In addition, the processing temperature is high (>160° C.). Furthermore, in order to efficiently decrease light reflection, a second processing bath that contains dangerous chemicals, such as hydrogen fluoride (HF) or fluoride, is required. Moreover, the volume of resultant pores cannot be controlled almost through the entire process.
In the meantime, devices such as a thin-film photovoltaic cell require a glass substrate. For a photovoltaic cell, it is required to increase the transmittance of light while decreasing as much reflectance as possible, i.e. improve the antireflection property. In other words, it is possible to increase the efficiency of the photovoltaic cell by decreasing as much reflectance as possible on the surface of the glass substrate. However, in order to improve the above-described antireflection property, the approaches of the related art merely include forming a coating having the antireflection property on a glass substrate or finding a coating composition that imparts antireflection effects and coating the glass substrate using this composition (e.g. see Korean Patent Application Publication Nos. 10-2010-51090 and 10-2010-19959). However, these approaches of the related art form a separate coating layer, resulting in several problems as described above.
When a vehicle windshield is fogged, sight can be obstructed, thereby causing difficulties in driving. Concerned with this problem, United States Patent Application Publication No. 2009-0239017 proposes realizing an antifogging function by disposing a film which has an antifogging property on the front surface of the windshield. However, this also requires a separate film layer, thereby creating the above-described problems.
According to the foregoing approaches of the related art as described above, the problems require formation of a separate coating layer made of a special coating material or use of harmful chemicals in order to impart glass with an antireflection property, an antifogging property and a self-cleaning property. Furthermore, any method for imparting these properties to one piece of glass is not disclosed at all.